California heating question
robertman
| Posted in Mechanicals on
I have a 1910 craftsman home in the Bay Area where we might need cooling 4 or 5 days year and it typically does not get too cold. We are redoing the basement (seismic) and attic (new roof, beefed up framing) and will install a solar PV system. I will most likely go to a heat pump water heater and a new heating system (presently have floor furnaces). I am thinking about a mini split system or a ducted heat pump system with the ducts in the attic. What are the advantages and disadvantages to both? The house is 3 bedrooms and about 1800 square feet. We also have a wood stove which we use but don’t want that to be primary heat source.
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Replies
Rob,
I strongly urge you not to install any ducts in your attic unless you are planning to create an unvented conditioned attic. Here is a link to an article with more information on this topic: Keeping Ducts Indoors.
The main advantages of ductless minisplits are:
1. They are more efficient than duct-based systems (because they don't require electricity for large blowers to push air through ducts) and
2. They don't raise that head-scratching question, "How will I install ducts in this old house that has no room for ducts inside the conditioned space?"
If you want ducted heat in every room, you might consider a ducted minisplit.
Before making any decisions about heating & cooling systems it's important to carefully calculate the loads, even with modulating equipment oversizing is a constant problem. CA Title 24 requires a Manual-J load calculation and limits the degree of ovesizing allowed. While HVAC contractors would happily perform those calculations, it's human nature for them to be overly conservative "just in case", and it's far better to do it yourself or hire an outfit who is only selling you the accuracy of their math, not equipment.
Installing ducts in an attic adds to both the heating and cooling loads by being outside of the fully conditioned space, and breaches the pressure boundary of the house, which would otherwise be fully air-sealed at the attic floor.
A guy in my office just installed ductless heating/cooling in a similarly sized circa 1920 bungalow in suburban Boston. With some insulation & air sealing improvements it was possible to get there with just three heads. While a mini-ducted mini-split would have perhaps worked better from distribution point of view, he couldn't find a competent mini-split installer willing to do the ducts. But the ductless proposals were all over the place. Had he not done the room-by-room load numbers himself he could have easily been led astray. The compromise ductless approach is a bit oversized, but not egregiously so. A modest amount of oversizing is good for ductless efficiency as long as it's still modulating rather than cycling on/off for most of the season. With grossly oversized equipment it would modulate rarely, which leads to lower comfort and slightly lower efficiency.